Fuse

ABSTRACT

The fuse comprises an electrical conducting wire which serves as a fusible link and which is connected between two terminals and immersed in a high dielectric strength liquid. The liquid and the wire size are selected so that the rated current can be carried by the fuse link with heat flux high enough to cause the liquid surrounding the fuse link to be in a state of nucleate boiling just under the point of maximum nucleate boiling heat flux.

United States Patent [191 Stover et al.

[ Nov. 26, 1974 FUSE [75] lnventors: Joe V. Stover, Fullerton, Byron P.

Meyers, Yorba', both of Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22} Filed: Aug. 6, 1973 [2]] Appl. No.: 385,941

[52] U.S. Cl. 337/166, 337/204 [51} Int. Cl. H0lh 85/04 [58] I Field of Search 337/166, 204, 277, 280

[56] References Cited UNITED STATES PATENTS 3.710295 1/1973 Staub 337/204 FOREIGN PATENTS OR APPLICATIONS 643.484 9/1928 France 337/277 Primary Examiner-J. D. Miller Assistant li.\'umim'rFred E. Bell Attorney, Agent, or Firm-Allen A. Dicke, Jr.; W. H. MacAllister. Jr.

l57l ABSTRACT The fuse comprises an electrical conducting wire which serves as a fusible link and which is connected between two terminals and immersed in a high dielectric strength liquid. The liquid and the wire size are selected so that the rated current can be carried by the fuse link with heat flux high enough to cause the liquid surrounding the fuse link to be in a state of nucleate boiling just under the point of maximum nucleate boiling heat flux.

3 Claims, 4 Drawing Figures Pmimwwvz I 3.851.290 v SHE 20F 2 Fig. 3.

22- SQ-H8 I; 25 l4 Hvps I C5 I Grid I I7 I a 27 Modulator I Q L I1 J 32 l IO 7" Fuse 1 23 24 I "I" Y I I 9e 1 l 40 92 jn2 FUSE BACKGROUND This invention is directed toa fuse, and particularly a fuse which has a link which quickly electrically opens,

. when the rated ac or dc current of the fuse is exceeded.

There are a number of requirements where an electrical load is supplied with operating current, and, when conditions cause an increase in current through the load, the current must be quickly terminated in order to prevent damage to the load equipment. When internal arcing occurs in the load equipment, the resultant increase in current must be quickly detected, and

the current supply electrically opened to prevent the values given above are found in the case of dc operated high transconductance gridded microwave tubes. Other types of tubes may allow energy levels up to 300 joules to be dissipated internally before permanent damage takes place.

The best-known prior art used to protect a dc operated microwave tube has been not to directly interrupt the flow'of current through the microwave tube, but to divert it around the microwave tube through a lower impedance path. This technique and the electronic circuitry required is variously called an energy diverter or crowbar.

In normal operation, some small value of resistance is required to be placed in series with a microwave tube to insure that the energy is successfully diverted through the lower impedance crowbar circuitry. During normal operation, the crowbar circuit isan open circuit in shunt with the microwave tube.

When the microwave tube arcs, the arc current is sensed and activates the necessary drive circuitry to fire the crowbar device itself. The crowbar device is usually a triggered gas gap, a triggered vacuum gap, 21 thyratron, or an ignitron. The crowbar device'becomes a conducting low impedance short circuit which diverts energy away from the microwave tube and simultaneously short-circuits the high voltage DC power supply until the input AC circuit breaker opens to remove all DC voltage.

A second technique that has been used is to place a high voltage electron tube in series with the microwave tube. In normal operation, the electron tube is either totally conducting or is functioning'as the traditional series regulating element in the series regulator circuit 1 pacity results in an increased amount of energy being to maintain a constant microwave tube operating voltfuse-linked wire, causing melting. As heat is added to the melted wire, the temperature rises and eventually reaches the boiling point. The temperature rises far above the normal boiling point, and the change from a liquid to a gas vapor occurs. The vaporization occurs with explosive violence. The conductivity of the vapor decreases by several orders of magnitude. Heavy current conductionstops due to thepressure wave front generated by the wire explosion. Therefore a region of decreased gas pressure occurs in the volume formerly occupied by the conductor or fuse element. If this pressure decrease is allowed tov attain a sufficiently low value for too long a time and the voltage build-up across the fuse is rapid due to Lenzs law, a restrike" will occur and current will again flow through a highly ionized gas plasma.

For further disclosure, see the article entitled A High Voltage, Quick-Acting Fuse to Protect Capacitor Banks, by H. Bruce McFarlane, in Exploding Wires, Plenum Press, Inc., New York, 1959-, pages 324 to 344, the entire disclosure of which is incorporated herein by this reference.

. The advantage of the exploding wire fuse is that, with proper design and excellent control of external circuit parameters, it is a rapid high voltage, current interrupting device.

An increase in normal fuse link current carrying carequired to flow to initiate fuse interruption, e.g., 0.27

grams copper fuse wire require about2,000 joules for interruption; 0.57 grams copper fuse wire require about 6,000 joules; and 0.70 grams copper fuse wire require about 12,000 joules.

The explosive arc-dischargeand radially-expanding shock wave causes a rarefaction of gas pressure in the vicinity of the wire. This results in reduced dielectric strength and increased gas discharge are restrike probability. Thus, the fuse container must be properly sized to reflect this shock wave back to the former position of the fuse wire before the voltage builds up and a gas discharge are restrike occurs. The rate of voltage buildup is dependent upon external circuit parameters over which a fuse designer has little control. Due to the explosive arc discharge, the fuse container must be structurally strong to perform the task of shock wave reflection. Increased normal fuse current carrying capacity requires a stronger fuse container. The exploding wire fuse always has some amount of restrike.

The exothermic fuse uses a bi-metallic composition material for the fuse link. The link is basically an aluminum core with a jacket of palladium over the aluminum. This combination has a positive temperature coefficient of electrical resistance and changes state when a critical temperature is reached. When this critical temperature is reached, the constituents alloy suddenly and exothermically, resulting in violent deflagration of the fuse link without the support of oxygen.

Theoretically, this is an ideal combination for fuses, and it has been used very successfully for certain fuse applications where the fuse link did not exceed 5 mils in diameter. Especially for high voltages, experience has proven that the fuse link does not clear and remain cleared. It is true that the link disrupts; however, the material so contaminates the surrounding shell that the fuse never clears. Another disadvantage in using an exothermic fuse is that, normally, the time to clear an exothermic device runs into the millisecond range. The

clearing time of the fuse is in direct relation to the rise time of the discharge circuit. The advantage of the exothermic fuse is that it operates successfully below 20 kilovolts.

The exothermic wire fuse has the following disadvantages: The diameter of the fuse element cannot exceed mils or restrike will occur at high voltage; The time required for faultcurrent to drop to zero is in the tens to thousands of microseconds. The time required for fault current to drop to zero is in direct relation to the rise time of the fault current. This results because the exothermic wire interruption process is initiated by heat alone. Restrike generally occurs above-about 25 KV DC.

SUMMARY I To aid in the understanding of this invention, it canrise and melting of the fuse link.

It is, thus, an object of this invention to provide a fuse which is capable of quickly opening a circuit, when the circuit current exceeds the rated fuse value. It is another object to provide a fuse wherein the fuse link operates in a liquid, with the liquid in a nucleate boiling state at the fuse link at rated current so that additional current causes film-boiling which results in a rapid in crease in temperature and melting of the fuse link. It is a further object to provide a fuse which rapidly opens the fuse circuit so that aminimum amount of energy is dissipated in the load before the fuse is opened.

It is a further object to provide a fuse wherein the fuse link operates in a liquid, with the liquid providing high dielectric strength properties during and after fuse link opening to prevent fuse restrike. It also is a further object to provide a fuse wherein no toxic byproducts circuit for testing the fuse of this invention.

DESCRIPTION The fuse of this invention is generally indicated at 40 in each of the figures. FIGS. 1 and 2 illustrate fuse. 40 in detail. Electrodes 42 and 44 are metallic and have suitable configuration for connection to the circuitry which is to be fused by fuse 40. Cylindrical connectors 46 and 48 are respectively formed on the outer ends of the electrodes as an example of connectors for clamp ing into appropriate busbar clamps. However, the electrodes could have flat structures on them for flat clamping if desired. Electrodes 42 and 44 are of a'lower resistance material, such as copper, and are spaced from each other to prevent arcing therebetween. Each of the electrodes comprises body 50, clamp ring 52 and connector 54. Connector 54 has a stud 56 extending from the rear thereof, on which body 50 is screwed. Clamp ring 52 is bolted against the rear face of connector 54 by means of screws 58 to define an annular groove 60 for retention of the tubing 62.

Tubing 62 forms the exterior housing of fuse 40 along the main part of its lengthyTube 62 is dielectric and is preferably fairly flexible to permit expansion of its contents without destructive degradation of tube. Tube-62 thus forms an outer jacket made of rubber-like materials, such as neoprene, Buna N or similar flexible dielectric materials of various synthetic polymer composition materials. Each end of tube 62 is provided with an integral clamp ring 64 which is engaged in groove 60 to length of fuse 40, and thus the mechanical characteris-' tics of the link holder are a function of the desired mechanical strength of the entire fuse. Thus, the properties of the materials together with the tube diameter and tube wall thickness are chosen for the desired strength. A plurality of holes 68 provide circulation to the interior of the link holder tube. At each end link holder 66 is attached by threads 70 to the electrodes. In the design shown, the threads are on a nipple which is positioned in a recess within the rounded nose of connector 54. The'shape of the rounded nose and the recessingof theend of the link holder aids in reducing corona.

The exterior surface of link holder tube 66, adjacent its ends and within the recess is plated with metal. The plated surface 72 makes-electrical contact with the interior surface of the recess in connector 54. Fuse link 74 is the link which loses its continuity upon over current. Fuse link 74 is preferably in the form of a cylindrical copper or silver wire. It is attached mechanically at its ends by lying against the plated surface 72 when the electrodes are screwed onto link holder 66 by way of threads 70. In this way, fuse link 74 is mechanically clamped within the recess in the nose of the electrode and against the plated surface on the link holder.

Link supports 76, 78, and are engaged in grooves in link holder 66.'These link supports engage the interior of tube 62, to provide support for the tube. The link supports are preferably in a form which has three spaced radial legs so that there is circulation within the interior oftube 62. The link supports do not divide chamber 82' in the interior of tube 62 into separate spaces. Furthermore, each link support has holes therein through which the fuse link 74 can pass so that the fuse link is supported intermediate its ends. Fuse link 74 may pass through some of the holes 68 in link holder 66, but the principle support of the fuse linkis in the main part of chamber 82 exteriorally of link holder 66. In order to provide the desired length of fuse link 74, the fuse link may take a sinuous path or a spiral path through chamber 82. It is desirable that the fuse link be as free as possible of lying against any adjacent surfaces, but that it lie throughout its length within the chamber 82 so that it can be surrounded by liquid in the chamber.

Chamber 82 is filled with a liquid 86. Liquid 86 surrounds fuse link 74 to cool the fuse link. Liquid 86 is a high dielectric strength liquid which has its vaporization change-of-state temperature above the maximum storage or operating temperature of fuse 40. Furthermore, the liquid is such that it can boil'and recondense at atmospheric pressure without chemical breakdown of the composition. Furthermore, the liquid is preferably such that substantial composition breakdown does not occur when a short-time spark arcs therethrough. Particular dielectric liquids belong to the Freon Brand and Fluorinert Brand liquids with Freon 113, Freon 114, and Fluorinerts FC-75, FC-77', FC-78, etc., are particular examples of a useful liquid. Fluorinerts are manufactured by Minnesota Mining and Manufacturing Company. The name Freon is a DuPont trademark for a group of halogenated hydrocarbons, usually based on methane, containing one or more flourine atoms. The various Freons are described in more detail in Handbook of Chemistry and Physics,.49th edition, 1968, at page E-28. However, suitable liquids are not limited to the Freons and Fluorinerts, but those liquids which are dielectric and which can safely boil and have an arc pass therethrough are equally suitable. These liquids are believed to. all belong to the class of nonpolar liquids. The class of non-polar liquids should be further limited by the characteristics that they are liquid at normal ambient temperatures from minus 50 to plus 150 F, are dielectric, are non-toxic, and have no substantial degradation when arcing passes therethrough.

The normal operating conditions of fuse 40 are that when it is either placed in a normal operating circuit, such as FIG. 3, or in a test circuit, such as FIG. 4, at rated current, the heat output of fuse link 74 into liquid 86 causes nucleate boiling of the liquid at the surface of the fuse link. The heat is caused by the FR heating of the fuse link at rated current. Rated current" is defined as being that amount of current which is the maximum desired to be passed by the fuse. Any excess over the rated current is expected to cause fuse 40 to act to open the circuit.

The rated current is defined by the resistance of fuse link 74, its thermal relationship to the liquid 86, and, to a small extent, upon the manner of heat rejection from the liquid 86 to the external ambient. The large external surface area of the fuse 40, which rejects heat to the external ambient is sufficient that its thermallylimiting interface is between the fuse link 74 and liquid 86. Considering the thermal characteristics and boiling characteristics of the liquid and the resistance and shape of fuse link 74, the structure is designed so that nucleate boiling in its upper range occurs at rated current. When the current through the fuse exceeds the rated current, the heat output of the fuse link 74, due to the increased l R loss changes the boiling mode at the surface of the fuse link from the nucleate boiling mode to the film boiling mode with a consequent substantial reduction in heat transfer coefficient. With the reduction in heat transfer coefficient, the fuse link temperature immediately rises with resultant liquefaction and vaporization thereof. Arcing between the terminated ends of the fuse link occurs, but the liquid quenches the arc to terminate current through the fuse.

More detail with respect to the characteristics of nucleate boiling and film boiling are found in the following two references: Advances in Heat Transfer, edited by T. F. Irvine, Jr. et al., Vol 7, Academic Press, New York, 1971, and particularly the article Heat Tansfer Near the Critical Point, by W. B. Hall, from pages 74 through 86 of that volume, including the references. The other reference is Fundamentals of Heat Transfer, by Kutateladze, translated from the second revised edition printed in Leningrad, 1963, by Scripta Technica, Inc. and edited by Robert D. Cess, Academic Press, New York, 1963. In this book, chapters 17 and 18, from pages 342 through 398, respectively describe heat transfer and boiling and critical heat fluxes in boiling. The references relating to these chapters are shown at pages 479 through 481 of'this book. These disclosures are incorporated herein in their entirety by this reference, including their references.

Thus, fuse 40 uses an electrical conducting link 74 connected between two terminals and immersed in a high dielectrical strength liquid 86. The liquid used is selected together with the link wire size such that the rated rms pulse current can be safely carried with the link remaining in its normal solid metallic state. Liquid 86 has a high dielectric strength and has its vaporization change-of-state temperature above the maximum operating or storage temperature. ln normal operation, link 74 is surrounded by liquid 86. The wire size'generally required is of small enough diameter that the heat energy dissipated in the wire at rated rms pulse currents is carried to the outside of the liquid container using the change of state properties of liquid 86 at the heat of vaporization. The wire size is so selected that the fuse wire heat flux is high enough at rated rms pulse currents to place the liquid surrounding the wire in a state of nucleate boiling just under the point of maximum peak heat flux. As the rms pulse current increases either slowly, due to an increase in operating duty cycle of the load, or due to an arc within the load to cause a rapid rise in peak current through the fuse link 74, the liquid 86 surrounding the link is driven into thermodynamic state of partial film boiling. In this state, the heat transfer from the surface of the wire is restricted by the partial transformation of the liquid to the gaseous state surrounding the wire causing the fuse wire 46 to experience a rapid rise in temperature due to reduction in heat transfer from the wire into the liquid. This fuse wire temperature increase causes the fuse wire electrical resistance to increase, causing further power loss in the fuse wire which must be dissipated thermally. This increased fuse wire power loss drives the wire into the film-boiling region where the wire is totally surrounded by a sheath of vaporized liquid or gas, with the resultant decrease in heat transfer from the wire to the liquid. This results in a thermally regenerative cycle in which the wire temperature is forced higher resulting in higher fuse wire resistance and hence higher fuse wire power dissipation until the point is reached where the metal of link 74 rapidly changes from a solid toa liquid sure, the fuse wire can be forced to go directly from a solid to a gas phase without liquefying.

When the metal of the wire link goes into a gas state, the resistance of the metal gas vapor becomes very high due to the surrounding partial pressure of high dielectric strength liquid in the gas state. This, coupled with the rapid dispersion of metal vapor into the liquid, causes an even higher electrical impedance and increased dielectric strength in the former region of the wire. The metal vapors and the high temperature gas are rapidly cooled by the surrounding mass of liquid, and the region formerly occupied by the fuse wire is replaced with a high dielectric strength liquid.

Upon proper selection of the liquid and fuse link wire length, restrike due to the rapid build-up of voltage caused by the rapid rate of current interruption can be totally avoided.

. Referring to FIG. 4, the test circuit 90 shown therein is used to test the fuse 40 of this invention. Capacitor- 92 and resistor 94 are serially connected between power line 96 and ground line 98. High voltage power supply 100, resistor I02, and switch 104 are serially I With switch 106 open, the closure of switch 104 p e r Examples of the structure and testing of several fuses are given below.

EXAMPLE I The first embodiment of fuse was formed using a rectangular plastic container open to the atmosphere in place of the cylindrical sleeve 56 with electrodes spaced to provide the fuse link 74 lengths shown in column B in Table l. The fuse link 74 was formed of cylindrical bare copper wire of the wire gauge shown in column A in Table 1. Liquid 86 was Fluorinert FC 78, manufactured by Minnesota Mining and Manufacturing Company. With a temperature of about 70 F surrounding the fuse 40, rated current through the fuse which causes heat dissipation from the fuse link near the top limit of nucleate boiling is shown in column C of Table I.

With the test circuit of FIG. 4 having resistor 94' having values shown in column H of Table I and having a capacitor 92 having the values shown in column G of Table I charged to the voltages shown in column F of Table I, the fuse links 74 shown in column B burned out at the peak currents shown in column I of Table i. The interruption times were as shown in column D with a total energy loss in the discharged circuit being indicated in column E.

TABLE 1 Test Results of the EXAMPLE 1 Embodiment of the Fuse A B C D E F G H 1 Link 74 Link 74 Rated Link Interrupt. Req. Fuse Cap. 72 Cup. 92 Resistor 94 Peak Link Wire Ga. Length Current. Time Energy lnitiul Capacity Resistance, 74 Current AWG inches A rms us Joules Voltage, .1.! ohms A 40 10.0 5.0 10.0 46.2 4.00 10.0 10.0 400 40 20.0 5.0 10.0 57.0 9.00 10.0 10.0 900 40 100 5.0 12.0 54.0 2710 5.0 10.0 2710 40 10.0 5.0 76.0 27.0 10.0 2700 44 10.0 2.7 4.0 17.x 90 10.0 10.0 900 44 10.0 2.7 15.0 7.0 27.0 5.0 10.0 2700 44 100 2.7 15.0 7.0 27.0 5.0 l0.0 2700 44 10.0 2.7 15.0 7.0 27.0 5.0 10.0 2700 44 9.0 2.7 540.0 6.00 4.75 155K 41.7 44 9.0 2.7 260.0 12.00 4.75 155.8 560 44 14.25 2.7 5K.0 2400 4.75 15514 116.0 44 14.25 2.7 60.0 i 24.00 475 155.8 109.0 44 14.25 2.7 48.0 77.6 29.00 4.75 155.11 160.0

mits power supply 100 to charge capacitor 92. Charg- EXAMPLE I] ing current is limited by resistors 94 and 102. When the desired charge is reached by control of power supply voltage, switch 104 is opened. Closure of switch 106 discharges capacitor 92 through fuse 40. Discharge rate is principally controlled by resistor 94, because resistor 108 is a low-value shunt resistor to permit meas-.

' urement of the current through fuse 40. A currentmeasuring device, such as oscilloscope 110, is connected in parallel to resistor 108. Volt meter 112, which may be another scope or the same scope as 110, if its provided with a dual beam, is connected to measure the voltage across fuse 40. When switch 106 is closed, capacitor 92 discharges through fuse 40 to raise its current above its rated value. Thereupon, the fuse opens the circuit. It has been previously determined that the existence of a steady-state current through fuse 40 below the rated value did not influence the manner in which the fuse acted upon currents above rated value. Thus, the test circuit illustrated in FIG. 4 provides all therequired information.

The second embodiment of fuse 40 was formed with a cylindrical sleeve 62 made ofNylon reinforced Tygon material and of 1.5 inches interior diameter, with electrodes spaced to provide the fuse link 74 lengths shown in column B of Table II. The fuse link 74 was'formed of cylindrical bare copper and silver wire as shown in column .1 and of the wire gauge, shown in column A of 1 The data shown in Table 11 was taken using the test circuit of FIG. 3.

To permit pulsed operation of the traveling-wave tube 10, a grid modulator 32 may be connected be- TABLE I1 Test Results of the EXAMPLE ll Embodiment of the Fuse A B C D E F G H l .1

Link 74 Link 74 Rated Link Interrupt Req. Fuse Capv 26 Cap. 26 Resistor 27 Peak Link Link 74 Wire Ga Length Current Time Energy lnitial Capacity 24 Resis- 74 Current Material AWG inches A rms us Joules Voltage 11F tance, ohms A 38 22.0 8.0 26.0 42.0 4.5 64.0 540 Silver 38 32.5 8.0 34.0 42.0 4.5 64.0 540 Silver 39 22.0 6.3 27.0 42.0 4.5 64.0 540 Silver 39 22.0 6.3 20.0 42.0 4.5 64.0 540 Silver 39 22.0 6.3 22.0 42.0 4.5 64.0 540 Silver 40 32.5 5.0 18.0 42.0 4.5 64.0 540 Silver 40 32.5 5.0 15.0 43.0 4.5 64.0 560 Silver 40 22.0 5.0 16.0 42.0 4.5 64.0 540 Silver 40 22.0 5.0 12.0 39.5 42.0 4.5 64.0 540 Silver 40 18.0 5.0 14.0 42.0 4.5 64.0 540 Silver 41 18.0 4.1 18.0 42.5 4.5 64.0 580 Copper 40 2L5 5.0 16.0,, A 42.0 4.5 64.0 540 Silver 41 21.5 4.1 18.0 42.0 4.5 64.0 540 Copper Referring to FIG. 3, the fuse 40 of this invention is shown in connection with a traveling-wave tube 10 having an electron-emitting cathode 12, a slow-wave interaction structure 14, and a collector electrode 16. An input connection 17 is disposed at one end of the interaction structure 14 to apply incoming microwave energy to the device, while an output connection 18 is positioned at the oppositeend of the interaction structure to facilitate removal of the amplifier microwave energy. Control grid 20 is interposed between the cathode l2 and the interaction structure 14 in order to control the flow of the electron stream and thereby facilitate pulsed operation of the traveling-wave tube 10. Although any linear electron beam microwave amplifier tube which it is desired to operate with a depressed collector potential may be employed, the example shown illustrates only non-depressed collector operation using, for example, tube 10 which may be a 55911 traveling-wave tube manufactured by Hughes Aircraft Company, Microwave Tube Division, Los Angeles, Califorma.

1n the TWT beam power supply circuit, power supply 22. which may be any regulated power supply having appropriate voltage and current ratings, is connected through Resistors 23 and 24 and fuse 40 to the cathode 12. The positive terminal of the power supply 22 may also be connected to a level of reference potential designated as ground in FIG. 3. When the aforementioned exemplary traveling-wave tube is employed, a power supply 22 may be selected which provides a DC output voltage E of 42.0 kv. at a current of 17.0 amps peak and 4.0 A rms.

The TWT beam power supply circuit includes a redundant crowbar 25 for back up protection, a capacitor 26 and resistor 27. The crowbar 25 is used to divert energy away from the TWT 10 in the event of a TWT arc and resultant failure of fuse 40 to interrupt. Resistor 27 is provided to limit the current out of capacitor 26 while resistor 24 and resistor 27 in series limit the available fault energy into the TWT from capacitor 26. Capacitor 26 functions as an energy reservoir to supply pulsed electrical energy to TWT l0.

Crowbar 25 is not required in a typical application using fully developed fuses 40, but was used only during fuse development.

tween the control grid 20 and the cathode 12 of the tube 10. The grid modulator 32 may comprise a pulse generator which, for the aforementioned exemplary traveling-wave tube and circuit components, provides an output terminal connected to the grid 20. The quiescent voltage level is 42.0 kv. with respect to ground and provides a voltage pulse at a level of -41 .4 kv. relative to ground for a duration of 40 microseconds. This circuitry is described in more detail in J. V. Stover et al., US. Pat. No. 3,369,188, the entire disclosure of which is incorporated herein by this reference.

It is seen that, when the load current through power supply 22 and thus between the slow-wave structure 14 and cathode 12 or grid 20 exceeds the rated current, as by arcing within tube 10, fuse 40 promptly opens the power supply circuit. As stated above, the current is limited so that little arcing power is discharged into tube 10 before the power supply is isolated, and thus destructive power levels in the tube 10 are not incurred. As shown in Table ll,fuse 40 is a particularly sensitive fuse which quickly opens the circuit when the current exceeds the rated fusc current, and thus is capable of protecting sensitive power supply or load equipment.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.

What is claimed is:

l. A fuse comprising:

first and second spaced electrodes, said first and second spaced electrodes each being metallic and configured for connection into an electric circuit;

a housing interconnecting said first and second elec- I trodes, said housing being made of resilient dielectric material in the form of a-tubing having first and second ends, said first and second electrodes being clamped on said tubing at said first and second ends to close the otherwise open ends of said tubing so that said tubing forms a tubular housing;

a dielectric link holder having first and second ends respectively secured to said first and second electrodes, said link holder being of substantially rigid material to hold said first and second electrodes in spaced position, said link holder being positioned within said tubular housing;

fuse link having first and second ends respectively secured to said first and second electrodes, said fuse link being of electrically conductive material and being supported by said link holder away from said tubular housing, said fuse link being made of electrically conductive material which is solid at ambient temperature and which melts above ambient temperature, and which has electrical resistance so that, when electric current passes therethrough, heat is generated within said fuse link with more heat being generated when more current passes therethrough;

dielectric fuse link cooling liquid around said fuse ried away by said liquid by nucleate boiling at rated current and heat is carried away from said fuse link by means of film boiling when above-rated current passes through said fuse link so that said fuse link rises in temperature above the melting point thereof when the current through said fuse link exceeds the rated current, said dielectric fuse link cooling liquid being retained by said tubular housmg.

2. The fuse of claim 1 wherein said link support isya tubing having transverse holes therethrough to permit fuse link cooling liquid to penetrate to the interior of said fuse link support tubing and said fuse link passes through at least one of said transverse holes in said fuse link support to support said fuse link away from the liquid is a non-toxic, non-polar dielectric which is liquid at ambient temperature. 

1. A fuse comprising: first and second spaced electrodes, said first and second spaced electrodes each being metallic and configured for connection into an electric circuit; a housing interconnecting said first and second electrodes, said housing being made of resilient dielectric material in the form of a tubing having first and second ends, said first and second electrodes being clamped on said tubing at said first and second ends to close the otherwise open ends of said tubing so that said tubing forms a tubular housing; a dielectric link holder having first and second ends respectively secured to said first and second electrodes, said link holder being of substantially rigid material to hold said first and second electrodes in spaced position, said link holder being positioned within said tubular housing; a fuse link having first and second ends respectively secured to said first and second electrodes, said fuse link being of electrically conductive material and being supported by said link holder away from said tubular housing, said fuse link being made of electrically conductive material which is solid at ambient temperature and which melts above ambient temperature, and which has electrical resistance so that, when electric current passes therethrough, heat is generated within said fuse link with more heat being generated when more current passes therethrough; dielectric fuse link cooling liquid around said fuse link for carrying heat electrically generated in said fuse link away from said fuse link, said liquid being in direct contact with said fuse link when rated current passes through said fuse link and heat is carried away by said liquid by nucleate boiling at rated current and heat is carried away from said fuse link by means of film boiling when above-rated current passes through said fuse link so that said fuse link rises in temperature above the melting point thereof when the current through said fuse link exceeds the rated current, said dielectric fuse link cooling liquid being retained by said tubular housing.
 2. The fuse of claim 1 wherein said link support is a tubing having transverse holes therethrough to permit fuse link cooling liquid to penetrate to the interior of said fuse link support tubing and said fuse link passes through at least one of said transverse holes in said fuse link support to supPort said fuse link away from the walls of said tubular housing.
 3. The fuse of claim 2 wherein said fuse link cooling liquid is a non-toxic, non-polar dielectric which is liquid at ambient temperature. 